The Nature of Minerals and Rocks as Materials - Materials Science for Structural Geology - page 9

polyphase rocks, and the variety of possible accommodating mechanisms that may
arise. This topic will be discussed later ( Chaps. 5 - 7 ) .
In respect of intergranular attachment, rocks present a wide spectrum as they
range from zero-porosity monomineralic polycrystalline bodies to granular masses
that are little more than consolidated soils. The intergranular region in rocks thus
shows great variety and is often of the utmost importance for mechanical prop-
erties. At the one extreme, it may consist of clean, intact grain boundary between
identical or differing phases, with structural properties akin to those of grain
boundaries in metals. At the other extreme, the grains are separated by a more or
less continuous layer of a cementing material, itself often polycrystalline but much
finer grained. Between these two extremes are boundaries containing precipitates,
films and voids, or cracks in various extents at the contact between the two grains.
1.3.2 Simple Grain Boundaries
While a clean, intact grain boundary between identical phases may be uncommon
in practice, the concept of it provides a point of reference for many discussions and
forms a basis for explaining properties that are insensitive to the presence of
impurities. The study of such grain boundaries has advanced considerably since
around 1970, the general trends being covered in the following reviews and col-
lections of papers; Bollmann ( 1970 , 1982 ), Hirth and Balluffi ( 1973 ), Smith and
Pond ( 1976 ), Ashby et al. ( 1978 ), Chadwick and Smith ( 1976 ), Johnson and
Blakely ( 1979 ), Priester ( 1980 ), Balluffi et al. ( 1981 ), Hahn and Gleiter ( 1981 ),
Clarke ( 1987 ), Doherty et al. ( 1997 ), Farkas ( 2000 ), Flewitt and Wild ( 2001 ). In
the present context, the succinct review of Balluffi et al. ( 1981 ) is especially
relevant. We shall now attempt to summarize briefly the main concepts that have
evolved. It should be recalled first that the complete geometric specification of a
planar segment of grain boundary involves not only the relative orientation and
translation of the two joining crystal lattices but also the orientation and position of
the grain boundary itself, requiring, in general, nine parameters. There have been
two main conceptual approaches to the analysis of the grain boundary structure,
the earlier one a formal geometrical approach with emphasis on periodicities and
dislocations, the other, more recent, an analytical or atomic approach derived from
computer modeling of atomic interactions, with an emphasis on vacancies.
The geometrical approach. Since the grain boundary is a region where the
influence of the two periodic structures of the grains overlaps or interacts, this
region, or ''core'' of the grain boundary, can itself be expected to have a periodic
structure in some degree. This structure is described with reference to the fol-
lowing periodic lattices:
1. The individual crystal lattices of the grains.
2. The CSL, coincidence site lattice (Friedel 1926 ; Kronberg and Wilson 1949 ):
Given the crystal lattices defined in each grain but extended through all space,